Communication systems are known to comprise mobile transmitters and receivers, such as in-car mobile or hand-held portable radios, hereafter referred to as communication units, as well as fixed transmitters and fixed receivers, such as base stations or controllers (fixed end). A typical message within such a communication system may begin with a mobile unit converting an audio signal into a digital data stream suitable for transmission over an RF (radio frequency) channel to either another communication unit or the fixed end. Such systems are often used by public safety institutions, such as local or federal law enforcement agencies. The existence of commercially available radio frequency scanners makes it possible for unauthorized parties to monitor the information transmitted within such a communication system. To reduce unauthorized eavesdropping, communication systems encrypt communications such that, without knowledge of the encryption method and a decryptor, the communications are unintelligible.
As is known, digital encryption methods use a reversible algorithm to introduce randomness into a digital data stream. An algorithm that randomizes digital data is called an encryptor; that which reconstructs the original data from the randomized data, a decryptor. An encryptor/decryptor algorithm typically utilizes dynamic parameters, often referred to as keys or key variables, to uniquely specify the nature of the randomness introduced to the digital data stream. Thus, only encryptors and decryptors utilizing an identical algorithm and key are capable of reproducing intelligible messages. An example of an encryptor/decryptor algorithm is the Data Encryption Standard (DES). Typically, each communication unit within a secure communication system Can store anywhere from one to twenty keys for use in encrypted communications. A communication unit's capacity for key storage is typically limited by the cost of the storage devices and protection technology required to safely maintain a set of keys.
The limited number of keys which i a secure communication unit may typically store precludes the use of many features commonly found in unencrypted communication systems. For example, if two users within a secure communication system wished to initiate a secure point-to-point communication with one another such that no one else in the communication system, in addition to those outside the communication system, could monitor their call, they would require the exclusive use, for an indefinite period of time, of at least one of the limited number of keys available to all of the communication units within the system. In very large secure systems containing hundreds or even thousands of communication units, such exclusive use of key variables is obviously inefficient. Furthermore, such a communication would have a degree of privacy but would not, in general, be completely secure in that other units within the system could monitor the communication by trying each of the limited number of keys until the conversation was rendered intelligible. To prevent this situation, a unique key variable would need to be available to each communication unit in the system. The number of unique keys that would need to be added to each communication unit to accommodate all possible point-to-point communications in a large system, as well as the additional processing required to determine which key is being utilized, is prohibitive. Therefore a need exists for a method which allows point-to-point communications to be accomplished within secure communication systems without the inefficient exclusive use of key variables or the addition of a large number of unique key variables in each communication unit.